The Australian Transport Safety Bureau (ATSB) is Australia's national transport safety investigator. The ATSB's function is to improve safety and public confidence in the aviation, marine and rail modes of transport. The ATSB is Australia's prime agency for the independent investigation of civil aviation, rail and maritime accidents, incidents and safety deficiencies.

Aviation safety investigations & reports

Fairchild Industries Inc SA227-DC, VH-DMI

The crew of the Fairchild Metro III aircraft, registered VH-DMI,
heard a loud bang shortly after application of full power during
the take-off roll. The crew immediately retarded both engine power
levers and noticed that the left engine exhaust gas temperature was
increasing so they shut down the left engine. When a passenger
advised that `smoke and fire' were coming from the left engine, the
crew discharged the fire bottle into that engine. The crew then
shut down the right engine and ordered an evacuation out the right
side of the aircraft.

An external examination by the operator's engineers found damage
to the left engine turbine blades and shrapnel damage to the
exhaust nozzle. The operator removed the engine and sent it to an
approved workshop for examination and repair. The Australian
Transport Safety Bureau (ATSB) did not attend the examination, but
requested that all damaged components be forwarded to the ATSB for
detailed examination.

After the engine was disassembled, the ATSB was advised that the
engine failure was precipitated by a failure of the turbine first
stage disc rotating air seal. The rotating air seal's outer rim was
missing and the downstream turbine components received damage
consistent with fragments of the rotating air seal passing though
the turbine. The failed rotating air seal, the first, second and
third stage turbine wheels and nozzle guide vane assemblies were
forwarded to the ATSB for further examination.

Engine and component history

The Allied Signal TPE331-12UHR-701G turboprop engine, serial
number P-70210, had accumulated 9,139.8 hours and 15,585 cycles
since new and 3,066.9 hours since overhaul. In May 1997, it had
Allied Signal service bulletin TPE331-72-2002 incorporated, which
detailed replacement of the inner baffle with a new inner baffle
part number 3108039-2. Service bulletin TPE331-72-2030, that
detailed replacement of the compressor interstage seal assembly
support, was incorporated in July 1999, at 6,073 hours, during
engine overhaul, After overhaul, the engine was installed into DMI
where it remained until the failure.

The rotating air seal, part number 3103839-3, serial number
2-23315-945, appeared to have been installed in the engine since
new as its time and cycles since new were identical to those
applicable to the engine. During the engine overhaul in 1999, it
was inspected in accordance with the requirements current at that
time and found serviceable.

The ATSB was advised that cracking of the rotating air seals had
occurred in the past, but that it was rare to see a cracked
rotating air seal on engines that have the requirements of the
engine manufacturer service bulletins TPE331-72-2002 and
TPE331-72-2030 incorporated.

Rotating air seal examination

The examination of the rotating air seal and other components
from the failed engine is detailed in the ATSB's technical analysis
report number 40/01. The examination revealed that the entire outer
rim of the rotating air seal had separated from the flanged
section. About seventy percent of the rim circumference was
recovered and most material from the outer ten millimetres of the
plate flange was lost.

One location, where the loss of material was substantially
greater, exhibited a short length of fracture showing evidence of
fatigue crack propagation. Heat tinting over the area of fatigue
indicted that it was present prior to the event failure. The seal
had no evidence of material or manufacturing anomalies.

Examination of the turbine components

The turbine disks and nozzle guide vane assemblies showed
evidence of random impact damage to the blade leading edges. The
damage was consistent with the separated pieces of the failed
rotating air seal passing through the turbine.

A copy of the ATSB's technical analysis report, number 40/01, is
available on the ATSB web site at or from the ATSB on request.

Engine manufacturer's action

The engine manufacturer reported a number of documented
in-flight shutdowns due to separation of the rotating air seal
plate rim. Their investigation into the events concluded that
cracking in the rim area was due to elevated rim operating
temperatures, primarily due to hot gas leakage from deteriorated
first stage stator assembly hardware. To alleviate the problem, the
manufacturer introduced service bulletins TPE331-72-2002 and
TPE331-72-2030 and revised the engine maintenance manual to improve
inspection of the relevant components at hot section inspection
with the intention of preventing hardware prone to gas path leakage
from returning to service.

The engine manufacturer advised that investigation into previous
failures of the rotating air seal concluded that the cracking in
the rim area was due to elevated rim operating temperatures,
primarily due to hot gas leakage from deteriorated first stage
stator assembly hardware.

To alleviate the problem, the manufacturer introduced two
service bulletins, TPE331-72-2002 and TPE331-72-2030 and revised
the engine maintenance manual to improve inspection of the relevant
components at hot section inspection with intention to prevent
hardware prone to gas path leakage from returning to service. The
requirements of the service bulletins were incorporated in this
engine in 1997 and 1999 respectively. The workshop that overhauled
the engine indicated that incidents of rotating air seal cracks
were rare on engines that had been modified in accordance with the
service bulletins, but they had no service data on the failed
seals.

The subject rotating air seal had accumulated over 6,000 hours
before the engine had the requirements of the service bulletins
incorporated. At that time, the seal was inspected in accordance
with the current requirements. However, the possibility that the
rotating seal failure was related to damage incurred during the
seal's prior time in service could not be excluded.

The investigation determined that fatigue crack propagation led
to separation of the rotating air seal outer rim from the flanged
section. The fatigue cracking was consistent with the air seal
being operated with elevated rim temperatures prior to the
incorporation of the engine manufacturer's service bulletins.

Fairchild Industries Inc., SA227-DC, VH-DMI

1. FACTUAL INFORMATION

1.1 Introduction

On 12 August 2001,
the take-off of a Fairchild Metro 23 aircraft (VH-DMI) was
discontinued as a result of the failure of the left engine. To
investigate the event, the Australian Transport Safety Bureau
carried out an examination of major components from the first,
second and third turbine stages of the affected engine.

Centre rotating air seal plate (p/no. 3103839-3, s/no.
2-23315-945)

First-stage turbine wheel (p/no. 3103897-10, s/no.
3-03229-1075)

Second-stage nozzle guide vane assembly

Second-stage turbine wheel (p/no. 3103927-5, s/no.
981217201126)

Third-stage nozzle guide vane assembly

Third-stage turbine wheel (p/no. 3103838-2, s/no.
0-01345-1837)

An earlier disassembly of the engine had found these components
damaged to varying degrees.

To further assist the examination, two additional rotating air
seal plates were supplied. Extensive radial cracking of the flange
section had rendered both of these items unserviceable.

1.2 Component history

The failed engine was an Allied Signal (Garrett)
TPE331-12UHR-701G, serial number P-70210. At the time of failure,
the engine had operated for a total of 9,140 hours and through
15,585 flight cycles. The following life information was also
provided for the rotating components.